Stupid rookie mistake. I wanted to take my 2006 Tahoe Q6 out on the lake this weekend so yesterday I began to summerize the boat. This boat has the 4.3L Mercruiser. I put the muffs on and started the engine. The engine turned over and after a moment of ruff idle, she smoothed out and began to purr. I turn around and there's 6 inches of water in the bilge. Oh Crap!!! I shut everything down and found water pouring out of a "freeze plug" hole on the side of the block. I felt physically sick. Apparantly I missed the water drain plug on the rear/right during winterization. I checked the oil dipstick but didn't notice any milky color. Took it to the Mercruiser mechanic who seemed more hopeful than me. He said he has seen three cases this year of freeze plugs that had to be replaced but the engine blocks weren't damaged. Nothing to do now but sit and wait and hope the block isn't cracked.